The present invention relates to a chemical pulping process for the production of cellulose pulps. More particularly, the present invention relates to a modification of conventional chemical pulping processes which improves yield, reduces steam requirements, and produces a pulp which is more easily bleached and which possesses improved brightness stability.
Chemical pulping processes are characterized in that the wood fibers are released by dissolving the lignin which binds them together. Because lignin and other non-cellullosic portions of the wood chips are removed in the process, chemical pulping processes typically provide yields of 40-50% based on the dry chips.
The rate limiting step in any pulping process is the penetration of the pulping chemicals into the chips. While there are several theories for chip impregnation in chemical pulping, one theory is that the liquor diffuses through the network of chip lumen and pits to remove the lignin. A typical wood chip contains several million fibers depending on species and chip size. Each fiber is a miniature cylinder. The middle of that cylinder is the lumen. The lumen of each fiber is connected to the lumen of adjoining fibers by openings or windows called pits. Liquor penetration is limited by the microscopic size of the lumen and pits through the chip. Liquor in the lumen must still penetrate the fiber wall to remove the lignin in the middle lamella. Permeability of the fiber wall is poor and a significant portion of the wall must be dissolved to make it porous. This has two negative aspects, namely, loss of pulp yield due to loss of carbohydrates and contamination of the fiber wall with lignin degradation products. The latter makes the fiber difficult to bleach.
Efforts to improve chemical pulping processes by modifying the chip structure to expand its surface area and facilitate liquor impregnation have been made. D. Lachenal et al, "Chip Destructuring Improves Kraft Pulping", TAPPI Proceedings--1984 Pulping Conf. pp 13-16, reports that destructing chips by passage through crush rollers reduces rejects. Nolan, in U.S. Pat. Nos. 2,904,460 (1959) and 3,912,102 (1965) teaches shredding the chips along the grain to facilitate chip impregnation. A hammer mill, attrition mill or crushing roll is used to shread the chip. Nolan states that a single fiber would be the most efficiently tailored chip from a purely theoretical standpoint, but this results in severe fiber damage and unacceptable loss in pulp strength. Nolan's preferred chip is a pin chip which ranges from about 2.3 to 6.7 mm in cross-section.
Chip destructuring as taught by Lachenal and Nolan does not form the open porous network which characterizes the chip used in the present invention and only marginally improves chemical pulping. Consequently it is not widely practiced in the paper industry.